The Stirling was designed by Short Brothers to meet an Air Ministry specification from 1936. When the preferred design from Supermarine had to be abandoned, the Stirling was ordered for the RAF. It entered service in early 1941 but had a relatively brief operational career as a bomber, being relegated to second line duties from late 1943, when other more capable four-engined RAF bombers, specifically the Handley Page Halifax and Avro Lancaster, took over the strategic bombing of Germany.

The Stirling was used for mining German port areas and new built and converted Stirlings fulfilled a major role as a glider tug and supply aircraft during the Allied invasion of Europe in 1944–1945.

In the 1930s, the Royal Air Force was interested primarily in twin-engine bombers. These designs put limited demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. Power limitations were so serious that the British invested heavily in the development of huge engines in the 2,000 horsepower (1,500 kW) class in order to improve performance. In the late 1930s, none were ready for production. The U.S. and USSR were developing bombers with four smaller engines, which proved to have excellent range and fair lifting capacity, so in 1936 the RAF also decided to investigate the feasibility of the four-engined bomber.

The Short S.31 half-scale replica used for aerodynamic tests of the Stirling design

The Air MinistrySpecification B.12/36 had several requirements. The bomb load was to be a maximum of 14,000 lb (6,350 kg) carried to a range of 2,000 miles (3218 km) or a lesser payload of 8,000 lb (3,629 kg) to 3,000 miles (4,800 km) (incredibly demanding for the era). It had to cruise at 230 or more mph at 15,000 ft (4,600 m) and have three gun turrets (in nose, amidships and rear) for defence.[2] The aircraft should also be able to be used as a troop transport for 24 soldiers,[3] and be able to use catapult assistance for take off.[2] The idea was that it would fly troops to far corners of the British Empire and then support them with bombing. To help with this task as well as ease production, it needed to be able to be broken down into parts, for transport by train.[4] Since it could be operating from limited "back country" airfields, it needed to lift off from a 500 ft (150 m) runway and be able to clear 50 ft (15 m) trees at the end, a specification most small aircraft would have a problem with today.

Initially left out of those asked to tender designs, Shorts were included because they already had similar designs in hand and they had ample design staff and production facilities. Shorts were producing several four-engined flying boat designs of the required size and created their S.29 by removing the lower deck and boat hull of the S.25 Sunderland. The new S.29 design was largely identical otherwise: the wings and controls were the same, construction was identical and it even retained the slight upward bend at the rear of the fuselage, originally intended to keep the Sunderland's tail clear of sea spray.

In October 1936, the S.29 was low down on the short list of designs considered and the Supermarine Type 317 was ordered in prototype form in January 1937. However it was decided that an alternative design to Supermarine was needed for insurance and that Shorts should build it as they had experience with four-engined aircraft. The original design had been criticized when considered and in February 1937 the Air Ministry suggested modifications to the original Short design, including considering the use of the Bristol Hercules radial engine as an alternative to the Napier Dagger inline, increasing service ceiling (28,000 ft) and reducing the wingspan.[5] Shorts accepted this large amount of redesign. The project had added importance due to the death of Supermarine's designer, Reginald Mitchell, causing doubt in the Air Ministry.[6] The S.29 used the Sunderland's 114 ft (35 m) wing and it had to be reduced to less than 100 ft (30 m), the same limit as that imposed on the P.13/36 designs (Handley Page Halifax and Avro Manchester). In order to get the needed lift from a shorter span and excess weight, the redesigned wing was thickened and reshaped. It is often said that the wingspan was limited to 100 ft so the aircraft would fit into existing hangars but the maximum hangar opening was 112 ft (34 m) and the specification required outdoor servicing.[4] "The wing span was limited by the Air Ministry to 100 ft"[7][8] The limitation was actually to force the designer to keep overall weight down.[9]

In June 1937 the S.29 was accepted as the second string for the Supermarine 316 and formally ordered in October.

Shorts built a half scale version as the S.31 (also known internally as the M4 – the title on the tailfin), powered by four Pobjoy Niagara engines, which first flew on 19 September 1938, piloted by Shorts' Chief Test PilotJ. Lankester Parker. Everyone was happy with the design, except that the take off run was thought to be too long. Fixing this required that the angle of the wing to be increased for take off. If the wing itself was modified, the aircraft would be flying nose down while cruising (as in the Armstrong Whitworth Whitley). Shorts lengthened the undercarriage struts to tilt the nose up on take-off, leading to its spindly gear which in turn contributed to many take off and landing accidents.[10] The Short S.31 was scrapped after a take off accident at RAF Stradishall, Suffolk in February 1944.

The first S.29, now given the service name "Stirling" after the Scottish city, flew on 14 May 1939 with four Bristol Hercules II radial engines. Upon landing one of the brakes locked, causing it to slew off the runway and collapse the landing gear. A redesign added much stronger and heavier struts on the second prototype. On its first sortie two months later, one of the engines failed on take off but the aircraft landed easily. From then on, the record improved and service production started in August 1940 at Shorts' Rochester factory. The area, which included a number of major aviation firms, was heavily bombed in the opening days of the Battle of Britain, including one famous low-level raid by a group of Dornier Do 17s. A number of completed Stirlings were destroyed on the ground and the factories were heavily damaged, setting back production by almost a year. Some production was moved to Austin Aero's factory at Cofton Hackett just south of Birmingham and the factory there eventually produced nearly 150 Stirlings.[11] From this point on, the Belfast factory became increasingly important as it was thought to be well beyond the range of German bombers. However, Belfast and the aircraft factory were subjected to German aircraft bombing during Easter week of 1941. To meet the increased requirement for its aircraft during the war, satellite factories near Belfast were operated at Aldergrove and Maghaberry, producing 232 Stirlings between them. In 1940, bombing damaged Supermarine's factory at Woolston and the incomplete Type 316 prototypes. The 316 was cancelled in November 1940 leaving the Stirling as the only B.12/36 design.

Instrument panel and controls of Stirling Mk I

Although smaller than both of the pre-war American "XBLR"-designation designs; the 149-foot wingspan, 35-ton loaded weight Boeing XB-15 and the enormous, 212-foot wingspanned, 79-ton loaded weight Douglas XB-19, and nearly-as-large Soviet experimental heavy bomber designs, the Stirling had considerably more power and far better payload/range than anything then flying from any UK-based aviation firm. The massive 14,000 lb (6.25 long tons, 6,340 kg) bomb load put it in a class of its own, double that of any other bomber. It was larger than the Handley Page Halifax and the Avro Lancaster which replaced it but both of these were originally designed to have twin engines. The Stirling was the only British bomber of the period to see service that had been designed from the start with four engines; the Avro Lancaster was a re-engined, stretched-wingspan Avro Manchester while the Halifax was planned to be powered by twin Vulture engines but was similarly re-designed to use four Merlins in 1937, as the problems with the Vulture engines became clear (a nasty habit of catching fire and spitting out connecting rods, sometimes within 10 minutes of being started).[12]

The design had nose and tail turrets (the latter was notable for the wide angles of fire) and included a retractable ventral ("dustbin") turret just behind the bomb-bay. This proved almost useless due to cramped conditions, with the added distraction that the turret tended to drop and hit the ground when taxiing over bumps. It was removed almost from the start and temporarily replaced by beam hatches mounting pairs of machine guns, until a twin-gun dorsal turret could be provided. This turret also had problems; it had a metal back fitted with an escape hatch which turned out to be almost impossible to use. The later Stirling Mk.III used a fully glazed turret (the same FN.50 as in Lancaster) that had more room and an improved view. Later Stirlings could also carry an improved, low-drag remotely controlled FN.64 ventral turret.[13]

Attention was paid to reducing drag – all rivets were flush headed and panels joggled to avoid edges – but camouflage paint probably negated the benefit. The wing was fitted with Gouge flaps similar to those of the flying boats.

The first few Mk.Is had Hercules II engines but the majority had 1,500 hp (1,100 kW) Hercules XIs. The Mk.III, introduced in 1943, was similar with the exception of the new dorsal turret and the improved 1,635 hp (1,200 kW) Hercules VI or XVI engines, which improved maximum speed from 255 to 270 mph (410 to 435 km/h).

Even before the Stirling went into production, Short had improved on the initial design with the S.34 in an effort to meet specification B.1/39. It would have been powered by four Bristol Hercules 17 SM engines, optimised for high-altitude flight. The new design featured longer span wings and a revised fuselage able to carry dorsal and ventral power-operated turrets each fitted with four 20 mm Hispano cannons; despite the obvious gains in performance and capability, the Air Ministry was not interested.

In 1941, Short proposed a new variant, the S.36,[14] which was nicknamed "The Super Stirling" in a company publication. This Stirling would feature a wing span of 135 ft 9 in (41.38 m), four Bristol Centaurus radials and a maximum takeoff weight of 104,000 lb (47,174 kg). The performance estimates included a speed of 300 mph (483 km/h) and a 4,000 mile (6,437 km) range, with a weapons load of 10,000 pounds (4,500 kg) over 2,300 miles (3,700 km), or 23,500 pounds (10,700 kg) over 1,000 miles (1,600 km). The defensive armament of the S.36 was to be ten 0.50 calibre machine guns, in three turrets. It was initially accepted for testing under Specification B.8/41 (written to cover it) and two prototypes were ordered but Arthur Harris, as commander of Bomber Command, felt that production would be too slow and would be better used to give the existing design improved Hercules engines, for a higher ceiling. Shorts were told in May 1942 that the Air Ministry would not be continuing the project and in August Shorts decided to terminate work.[15]

Pilot accounts generally report that, once airborne, the Short Stirling was a delight to fly, surprisingly manoeuvrable for such a large aircraft and without any vices. The shortcomings of the aircraft in terms of lower operational altitudes and limited range are largely forgiven in pilot autobiographies.[16] The Stirling did, however, exhibit some vicious flying characteristics during takeoff and landings.

As a class, the large and heavy four-engined tail-wheeled bombers such as the Short Stirling, Handley Page Halifax, Avro Lancaster and Boeing B-17 Fortress could be a handful on takeoff and landing and especially so for the relatively young and inexperienced new pilots who formed the vast majority of the expanding Commonwealth and American air forces. Later heavy bomber designs such as the Consolidated B-24 Liberator and Boeing B-29 Superfortress used a nose-wheel (tricycle) configuration as did every[citation needed] successful four engined commercial aircraft in the post-war years. Tricycle geared aircraft are easier to control on takeoff, landing and during taxing, and also make for easier cargo loading and servicing as the cabin, engines and other systems are closer to the ground.

The Short Stirling had particularly challenging flying characteristics on takeoff and landing, even in comparison with other tail-wheeled contemporaries. After a series of serious accidents and total aircraft losses involving uncontrolled ground loops on takeoff, the Royal Air Force implemented a special training and certification program for all prospective Stirling pilots. Proper takeoff technique involved feeding in right engine throttle during the initial 20 seconds of the takeoff run until the rudder became effective for control. If all four throttles were advanced simultaneously, the aircraft would swing to the right, become uncontrollable and often collapse the landing gear which could be disastrous if the aircraft was loaded with bombs and fuel.[17]

On flare-out for landing, the Short Stirling exhibited a tendency to suddenly stall out and "drop like a stone" to the runway. With such a heavy aircraft, a "dropped" landing could cause serious structural damage.[17] During World War II it was not unknown for "dropped" landings to render Stirlings or other large four-engined bombers unairworthy and suitable only for parts.

Operational status was reached in January 1941, by No. 7 Squadron RAF. The first three Stirlings flew a mission on the night of 10/11 February 1941 against fuel storage tanks at Vlaardingen near Rotterdam and from the spring of 1942, the bomber started to be used in greater numbers.[18][19] From May 1943, raids on Germany were conducted by over a hundred Stirlings at a time.[20]

Despite the "disappointing performance" at maximum altitude,[21] Stirling pilots were delighted to discover that, due to the thick wing, they could out-turn the Ju 88 and Bf 110nightfighters they faced. Its handling was much better than that of the Halifax and some preferred it to the Lancaster. Based on its flight characteristics, Flt Lt Murray Peden (RCAF) of No. 214 Squadron RAF described the Stirling as "one of the finest aircraft ever built".[22]

Another consequence of the thick wing was a low ceiling and many missions were flown as low as 12,000 ft (4,000 m). This was a disadvantage on many raids, notably if crews were attacking Italy and had to fly through (rather than "over") the Alps. When Stirlings were on combined operations with other RAF bombers which could fly higher, the Luftwaffe concentrated on the Stirlings. Within five months of being introduced, 67 out of the 84 aircraft delivered had been lost to enemy action or written off after crashes.

The Stirling's maximum bomb load could be carried for only a short distance of around 590 miles. On typical missions deep into Germany or Italy a smaller 3,500 lb (1,590 kg) load was carried, consisting of seven 500 lb (227 kg) GP bombs. This was the sort of load being carried by the RAF's medium bombers such as the Vickers Wellington and by 1944 the de Havilland Mosquito. Perhaps the biggest problem with the design was that although the bomb bay was large at 40 ft long (12 m)[23] it had two structural dividers running down the middle, limiting it to nothing larger than the 2,000 lb (907 kg) bomb. As the RAF started using the 4000-lb (1,815 kg) "cookies" and even larger "specials", the Stirling became less useful. The Handley-Page Halifax and especially the Avro Lancaster offered better performance (the Lancaster could carry twice the Stirling's bombload over long distances and was at least 40 mph faster while having an operating altitude of about 4,000 ft higher[24]) so when they became available in greater numbers from 1943, it was decided to relegate Stirlings to secondary tasks.

By December 1943 Stirlings were being withdrawn from frontline service as bombers, increasingly being used for minelaying outside German ports ("Gardening" missions), electronic countermeasures and dropping spies deep behind enemy lines at night (through the unused ventral turret ring). Also at that time, there arose a need for powerful aircraft to tow heavy transport gliders such as the General Aircraft Hamilcar and Airspeed Horsa; the Stirling fitted this role admirably. In late 1943, 143 Mk.III bombers were rebuilt to the new Mk.IV series specification (without nose and dorsal turrets), for towing gliders and dropping paratroops, as well as 461 Mk.IVs being built. They were used in the Battle of Normandy and Operation Market Garden. Stirlings were also used in Operation Glimmer on 6 June 1944 for the precision-laying of patterns of "window" (later known as "chaff") to produce radar images of a decoy invasion fleet.[25] From late 1944, 160 of the special transport variant Mk V were built, which had the tail turret removed and a new opening nose added, most of these being completed after the war.

In service with Bomber Command, Stirlings flew 14,500 sorties, dropped 27,000 tons of bombs, and lost 582 in action with another 119 written off.

The Stirling is listed in the appendix to the novel KG 200 as one flown by the German secret operations unit KG 200, which tested, evaluated, and sometimes clandestinely operated captured enemy aircraft.[26]

Six Stirlings were purchased by the Egyptian Air Force for use in the 1948 Arab Israeli War, forming the 8th Bomber Squadron. These flew a number of air raids on Israeli targets in the 1948 war, one of their number being lost either as a result of an accident or sabotage. The remaining five appear to have been scrapped or retired by 1951.[27]

Trans-Air, later known as Air Transport (Post-war civilian use, a total of 10 planes, 9 of which went on to the Egyptian Air Force. The 10th (OO-XAC, ex-PK172) crashed during operations in Kunming, China)[28][29]